Sensor for measuring gas permeability of a test material

ABSTRACT

A sensor for measuring gas permeability of a test material, comprising: an electrically conductive sensing element that comprises a water and/or oxygen sensitive material, wherein the reaction of said material with water or oxygen when the sensing element is contacted with water and/or oxygen results in a change in the electrical conductivity of the sensing element, and two electrodes electrically connected to the sensing element.

PRIORITY CLAIM AND CROSS-REFERENCE TO RELATED APPLICATION

This application is filed under the provisions of 35 U.S.C. §371 andclaims the benefit of priority of International Patent Application No.PCT/SG2004/000075 filed Mar. 31, 2004. The disclosure of saidInternational Patent Application is hereby incorporated herein byreference, in its entirety.

The invention relates to a sensor for measuring gas permeability of atest material, a method of producing a sensor, and a gas permeabilitymeasurement system. The invention also relates to a method ofdetermining gas permeability through a test material.

The emergence of new technologies in the electronics and biomedicalfields in recent years has prompted the parallel development of newmaterials and structures.

One example of such a technology is the flexible flat panel displaywhich uses flexible organic light emitting devices (FOLED). Each FOLEDcomprises materials and structures having electrical, mechanical andoptical properties that allow for large area displays. However, FOLEDarchitectures comprise organic electroluminescent materials and cathodesthat are susceptible to damage from reaction with water vapour andoxygen in the atmosphere. Therefore, FOLED displays require high barriersubstrates, sealants and encapsulation materials, which are extremelyimpermeable to water and oxygen in order to attain the industrystandards specified for its minimum lifetime.

Another technology which requires water and oxygen sensitive materialscan be found in the area of memory storage, particularly in the harddisk drive industry. The material used in hard disk platters comprisemetal or metal oxide which can be degraded by oxygen. In order tomaintain the integrity of magnetic data, packaging structures andmaterials must provide low levels of gas permeation in order to preventthe degradation of the seal and metal oxide in the platter.

Other examples of applications which require the use of materials thatprovide gas barrier properties include hermetically sealed packagesfound in food, drug and biological sample packages. In addition,sealants, plastics and composite materials are also often required tohave gas barrier properties.

More recently, applications have been developed that require materialsand structures possessing very low gas permeability. These materials andstructures have in turn necessitated the use of sensitive measurementinstruments, in particular gas sensors, which can assess gas permeationproperties at very low gas permeation levels.

One class of sensors rely on detection methods which do not involvechemical reaction with the target gas. U.S. Pat. No. 6,067,840 issued toTexas Instruments Inc. discloses an optical infrared (IR) gas sensor.The differential absorption between two IR sources, each directed to atarget gas and a reference gas, is used to determine the concentrationof a gas being monitored.

U.S. Pat. No. 6,460,405 issued to MOCON, Inc. discloses a gas sensor inwhich a test specimen is exposed to a chemically inert tracer gas, suchas helium or carbon dioxide. A tracer gas detector is provided tomeasure the flow of tracer gas through the specimen and the measurementsare correlated to the gas permeability of the test specimen.

U.S. Pat. No. 6,567,753 discloses a sensor for determining barrierproperties of a barrier coating with respect to a plurality of fluids. Adual-response acoustic wave transducer is coated with the barriercoating and exposed to the plurality of fluids. The permeation of thefluid or dissolution of the coating is subsequently measured usingacoustic wave and optical detection measurements.

Another class of sensors utilise sensing elements which react with thetarget gas. Kumar et al. (Thin Solid Films 417, 2002, 120-126) disclosesa low moisture permeation sensor that relies on the optical measurementof corrosion on the surface of calcium films. The calcium film isinitially a highly reflective metallic surface. As water vapour andoxygen progressively react with the calcium film, the reflective surfacegradually turns into an opaque film. The test specimens are photographedat regular intervals to monitor the change in transmission property ofthe calcium films. By subjecting the photographs of the calcium film toanalysis using image analysis software, the change in optical propertiesof the calcium film is correlated to the flow rate of water vapour intothe encapsulation structure. This type of sensor is also disclosed in G.Nisato et al., Evaluating High Performance Barrier Films, InternationalDisplay Workshop, October 2001.

One drawback of the currently available gas sensors is their inabilityto measure gas permeation at a sufficiently high level of sensitivityrequired for assessing very low gas permeability materials. Forinstance, the industry standard specified for the lifespan of FOLEDdevices is more than 10,000 operating hours. In order to attain thislifespan, oxygen and water vapour transmission rates through theencapsulation structures of FOLED devices should be below 10⁻⁵ g/m²/dayand 10⁻⁶ cc/m²/day respectively (at 38° C. and 95% relative humidity).However, available instruments have sensitivity limits of around 10⁻³g/m²/day for water and about 10⁻³ cc/m²/day for oxygen.

Another drawback of the currently available gas sensors is that theservice temperature is limited to moderate temperatures of typicallyabout 50° C. With a low temperature limitation, performance tests cannotbe carried out under conditions for bringing about accelerated gaspermeation, thereby resulting in long test durations which areuneconomical to carry out.

Accordingly, it is an objective of the present invention to overcomedisadvantages of currently known sensors. It is a further objective ofthe invention to provide a sensor which has high sensitivity, goodspatial and time resolution, high service temperature, but is stilleconomical to manufacture and use. This objective is solved by a sensorand a respective method of producing the sensor having the features asspecified in the independent claims. Such a sensor of the presentinvention is a sensor for measuring gas permeability of a test materialcomprising:

an electrically conductive sensing element that comprises a water and/oroxygen sensitive material, wherein the reaction of said material withwater or oxygen when the sensing element is contacted with water and/oroxygen results in a change in the electrical conductivity of the sensingelement, and

two electrodes electrically connected to the sensing element.

The invention is based on the discovery that gas permeation measurementsthat are performed on materials and structures such as low gaspermeability materials can be made significantly more sensitive bymonitoring the electrical and, optionally, noise properties of the gassensor during the course of the measurement. Accordingly, the gas sensorof the present invention comprises an electrically conducting sensingelement that is also sensitive either to water or oxygen alone, or toboth water and oxygen. In addition, the sensing element functions as anelectrical resistor which has optical, electrical and noise propertiesthat change as it gradually reacts with water and/or oxygen during ameasurement test. This change permits the rate of permeation (understeady state conditions) of moisture through the low gas permeabilitypolymer substrates to be determined directly by physical evidence ofchemical reaction of water vapour with a sensor, for example by opticalmeasurement techniques. Alternatively, other methods such as 1/f noisespectrum measurement and resistance of the sensor can be employed tomonitor sensor degradation. From these measurements, the rate of changeof calcium thickness with respect to time and thus gas transmissionrates across the test specimen may be derived.

The use of the sensor of the present invention in gas permeationmeasurements has several advantages. Firstly, the present sensor canmeasure gas transmission rates with high sensitivity of better than 10⁻⁸g/m² day, and provides better time resolution as well as low percentageof error. Accordingly, it is suitable for assessing the gas permeationproperties of polymer substrates, barrier coated films, or multi-layerbarrier stacks which have low gas permeability properties. Secondly, thepresent invention provides for the measurement of combined transmissionrates of oxygen and water vapour within a single test, meaning that themeasurement of water vapour transmission rate (WVTR) and oxygentransmission rate (OTR) can be carried out in a single instrumentconsole. Thirdly, transport coefficients such as the permeabilitycoefficient, diffusion coefficient and solubility coefficient can besimultaneously determined in one test.

In the context of the invention, the term “target gas” refers to the gaswhich a test material or test structure is exposed to in order tomeasure the rate of transmission of the gas through that material orstructure. The term includes individual gases such as oxygen or watervapour, and simple or multi-component mixtures thereof with nitrogen,carbon dioxide, hydrogen and sulphur dioxide, for example. Examples ofmulti-component mixtures include air and exhaust gases.

The terms “test material”, “test structure” and “test specimen” are usedinterchangeably to refer to the material/structure that is being testedfor its gas permeability properties using a gas sensor of the presentinvention.

The sensing element can comprise any suitable electrically conductivematerial which is sensitive to oxygen and/or water. This means that thematerial can be sensitive either to water alone, or oxygen alone, orboth water and oxygen. Suitable materials include metals, metal alloys,metal oxides, conductive polymers, as well as mixtures and combinationsthereof.

In principle, all metals that can react with water and oxygen can beused as the sensing element or within the sensing element. Such metalsinclude highly reactive metals such as the Group I elements (forexample, sodium and potassium), moderately reactive metals such as GroupII elements (magnesium, calcium, barium) and transition metals such asiron, tin and chromium. Particularly suitable metals are calcium andmagnesium. Apart from being reactive towards water and oxygen, they alsocan be readily processed into any suitable shape and dimension, such asblocks, strips or thin films.

Examples of conductive polymers which are contemplated for use within oras the sensing element include conjugated organic polymers, conjugatedmetallopolymers (inorganic polymers) and redox polymers. Examples ofuseful conducting polymers include polyaniline, polypyrrole,polythiophene, polyacetylene, poly-p-phenylene, and polyvinylpyridine,thiophene-bipyridine copolymers, polypyridine, polybipyridine, andorganometallic polyphenylenes.

Examples of metal oxides which are contemplated for use as or within thesensing element include VO₂, CrO₂, MoO₂, and LiMn₂O₄; transparentconductive oxides such as cadmium stannate (Cd₂SnO₄), cadmium indate(CdIn₂O₄), zinc stannate (Zn₂SnO₄ and ZnSnO₃), and zinc indium oxide(Zn₂In₂O₅). No particular restriction is placed on the crystal structureof an oxide that is used, which can be either crystalline,nanocrystalline, or amorphous, for example.

The sensing element can also be formed from mixtures and combinations ofthe above materials. For example, it is possible to form a sensingelement composition from a solution containing a suitable organicpolymer and metallic particles, such as iron or calcium particles.

In order to carry out a measurement test, the sensor can be deployed inseveral ways. For example, the sensor may be embedded within or formed(deposited) on a surface of the test material. Alternatively, if anencapsulated environment is to be assessed, the sensor can be placedwithin the encapsulated environment. The test material and theaccompanying sensor is then exposed to an atmosphere containing thetarget gas which is reactive towards the sensing element. Apredetermined quantity of the sensing element is used for themeasurement, and the time taken for the sensing element to be partiallyor completely reacted can be determined. One possible way to do so is tomeasure the change in current flow over a period of time, and thencalculate the projected time taken for the sensing element to be fullyreacted. Another possible way is to monitor the ceasing of current flowthrough the sensor. For example, when the current flow ceases, thesensing element may be assumed to have reacted completely.

In principle, the sensing element is able to operate at any thickness,as long as a sufficient period of time has elapsed for a sufficientquantity of the target gas to diffuse through the test material and toreact with the sensing element. However, when measuring low levels ofgas permeation, the sensing element may comprise of small quantities ofthe sensitive material in order to keep the measurement test durationwithin a reasonable length of time. For this reason, the thickness ofthe sensing element can be designed to be in the micrometer or nanometerrange. For the purpose of measuring low gas permeability materials, thesensing element may have thicknesses ranging from 10 nm to 10 microns,preferably 50 nm to 1 μm and in some cases more preferably between 120nm to 500 nm. The other dimensions of the sensing element such as lengthand width may be varied according to the size, shape, type andrequirements of the test sample.

Some design principles can be applied to the present the sensingelement. Firstly, the sensing element should preferably possesselectrical properties that are low or close enough to the bulkproperties of the sensing material. Secondly, the minimum thickness (H),length (L) and width (W) of the sensing elements can be optimised withmeasured electrical properties. In this regard, the area of the sensingelement (L×W) may depend on the test substrate size as well as theexperimental design for carrying out the measurement test. For example,sufficient space needs to be allocated for the encapsulation and forconductive tracks. It should be noted that one or more sensing elementsmay be used with a single test substrate. Each sensing element caneither be in the shape of a square, rectangle, multiple stripes or anyother desired shape in order to detect the permeability at the differentlocations on the item being tested. Furthermore, the minimum size (area)of the sensing element can also be influenced by factors such as theelectrical properties of the sensor, while the maximum size (area) canalso be influenced by factors such as substrate dimension andexperimental design.

In one specific embodiment in which calcium is used as material for thesensing element, an optimal dimension for the calcium sensing element isabout 1 cm length, about 2 cm width & about 150 nm thickness. Themeasured resistance of the sensor formed with this sensing element isabout 0.37 Ω-cm, and is thus close to the bulk calcium sensing element'sresistance of 3.41 Ω-cm. The two metal tracks that are used aselectrodes in this embodiment have a dimension of about 2 cm by 2 cm.The cover glass (about 3.5 cm×about 3.5 cm) used in this embodiment forthe encapsulation and rim-sealing (adhesive) width is about 2 mm.

In the invention, the sensing element is provided with electrodes(electrical connectors) to provide a means to couple the sensing elementto a voltage source. Electrodes can assume any suitable shape, size orform, such as a conventional insulated copper wire, metal plates or thinfilm conductive tracks. The electrodes can comprise any type ofelectrically conductive material, commonly used materials being metals,metal oxides or mixtures thereof. The materials used here are preferablyunreactive towards reactive gases comprised in the target gas, such aswater and oxygen, under test conditions so that the accuracy of thesensor is not adversely affected during the course of the test.Nevertheless, metals that are reactive with these reactive gases may beused if their reaction rate with such gases is much slower than thereaction rate of the sensing element with the reactive gases. If suchmetals are used, it is also possible that they are provided with aprotective material prior to such use. This can be achieved, forexample, by surface treating the metal with an inert protective coating.This ensures that the electrical resistance of the electrodes is notaffected during the course of the test.

If the sensor is used to test low gas permeability materials, it ispreferable that the electrodes comprise materials that are suited foruse with deposition equipment that are used in the manufacture of suchsensors. Suitable materials include as metals, metal alloys, and metaloxides. Examples of metals that are suitable include silver, copper,gold, platinum, titanium, nickel, aluminium, lead and tin and theiralloys. Alloys such as aluminium alloys, or iron/nickel, iron/chromiumor iron/cobalt alloys can also be used in the invention. In addition,oxides with good electrical conductivity such as indium tin oxide,aluminium zinc oxide and indium zinc oxide, can be used as well. Anymixture or combination of these materials may also be used.

The electrical connection between the two electrodes and the sensingelement can be formed using any suitable means of connection. Forexample, it is possible to adhere the electrodes to a surface of thesensor using a conductive tape or to solder the electrodes to thesensing element using a low resistance soldering metal e.g. tin.Alternatively, conductive bond pads can also be applied onto the sensingelement on which the electrodes can be connected.

In one embodiment, the sensor comprises a base substrate that supportsthe sensing element. A base substrate facilitates the prefabrication,packaging, handling and transporting of the sensor. For instance, thebase substrate can provide a suitably large surface area on whichauxiliary electrical connections to the electrodes, e.g. bond pads, canbe formed. The base substrate can take a variety of forms, such as a PVCboard, a PET sheet or an adhesive film that is attachable to a testspecimen.

In principle, any material that is inert towards any reactive gaspresent in the target gas, and sufficiently permeable to an extent thatallows testing to be carried out, may be used as the base substrate. Inthe context of this invention, an inert material refers to any materialwhich is not reactive towards water vapour or oxygen which is present inthe target gas. This material can have any suitable gas permeability,e.g. a porous material or a low permeability material. One suitableclass of materials which can be used to support the sensor and canexhibit a wide range of gas barrier properties are polymers.

Polymers which are contemplated for use in the base substrate in thepresent invention include both organic and inorganic polymers. Examplesof organic polymers which are suitable for forming the base substrateinclude both high and low permeability polymers such as cellophane,poly(1-trimethylsilyl-1-propyne, poly(4-methyl-2-pentyne), polyimide,polycarbonate, polyethylene, polyethersulfone, epoxy resins,polyethylene terepthalate, polystyrene, polyurethane, polyacrylate, andpolydimethylphenylene oxide. Microporous and macroporous polymers suchas styrene-divinylbenzene copolymers, polyvinylidene fluoride (PVDF),nylon, nitrocellulose, cellulose and acetate may also be used. Examplesof inorganic polymers which are suitable in the present inventioninclude silica (glass), nano-clays, silicones, polydimethylsiloxanes,biscyclopentadienyl iron, polyphosphazenes and derivatives thereof. Thebase substrate may also comprise a mixture or a combination of organicand/or inorganic polymers. These polymers can be transparent, semitransparent or completely opaque. If optical measurements on the sensingelement are to be carried out in conjunction with the electricalmeasurement as described hereinafter, it is preferable to use a basesubstrate material that also provides a suitable level of reflectivityto aid in the observation or photographing of the sensing element.

As the base substrate may possess some degree of resistance to gas flowand would thus have an impact on gas permeation readings, gas permeationproperties of the base substrate should preferably be characterisedprior to use, so that the final measurements can be appropriatelyadjusted to take into account the effect of the base substrate. Where alow gas permeability material is used as the base substrate, the targetgas may take a longer time to permeate through the base substrate andthus requires a longer test duration. Alternatively, if highly permeablematerials, e.g. cellophane, are used as the base substrate, there willbe relatively less resistance to gas flow.

In a further embodiment, the base substrate further comprises a barrierlayer. In this context, a barrier layer is generally known in the art tobe made from one or more layers of materials such as barrier polymers,metals or ceramics that can be used to separate a system or an article,e.g. an electronic component or food, from an environment (cf. U.S. Pat.No. 6,567,753—col. 1, line 17 to 21 or claim 5). When the base substratecomprises such a barrier layer (which preferably has a low permeabilitytowards the target gas), the base substrate and the barrier layer eachprovides different types of functions in the sensor. For example, thebase substrate itself may function only as a supporting structurewithout providing much resistance to the target gas, while the barrierlayer provides the desired level of gas resistance. The respectivematerials chosen for the barrier layer and base substrate should eachhave a correspondingly suitable level of gas permeability necessary forthe use of the sensor of the invention.

It is contemplated that the barrier layer can be arranged or positionedin several ways in this embodiment. In one example, the barrier layermay be present as a single layer or as a laminated sheet or coatingbetween the sensing element and the base substrate. Alternatively, thebarrier layer can be a layer located within the base substrate. It isalso further contemplated that the barrier layer can be located at thebottom of the base substrate, i.e. diametrically opposed to the sensingelement. Typically, if a barrier layer is present, it consists orcomprises the material to be tested, i.e. the permeability of whichtowards a target gas is to be measured.

In another embodiment, the base substrate consists solely of the barrierlayer, meaning that the sensing element is formed directly onto only abarrier layer. This embodiment provides for a means to test thepermeation characteristics of the barrier layer using the sensor of thepresent invention. In this manner, the present embodiment can also servefor example, as a ready-made unit for use in kits to simulate thepermeation processes through the barrier layer.

Suitable materials for forming the barrier layer can comprise inorganicor organic materials. Inorganic materials that are particularly suitableinclude metals (e.g. aluminium, iron, tin), metal oxides (e.g. Al₂O₃,MgO, TiO₂), ceramic oxides, inorganic polymers, organic polymers andmixtures and combinations thereof. Examples of respective inorganicpolymers include organic-inorganic polymers, metal chelate coordinationpolymers, and completely nitrogen based inorganic polymers. Specificexamples are glass, silicones, polydimethylsiloxanes,biscyclopentadienyl iron, polydichlorophosphazene and derivativesthereof, for example. Suitable organic materials include organicpolymers such as acrylic-based polymers, polyimide, epoxy resins,polyalkylenes derivatives such as crosslinked polyethylene, polyethyleneterephthalate, polystyrenes, polyurethanes, polyacrylates,polycarbonates, and polyethersulfones. Particularly suitable organicpolymers having a suitable level of permeability and stability includePET, polycarbonate and polyethersulfone. The barrier layer can be asingle layer film or a multi-layer stack. In the case of a multi-layerfilm where independent layers of inorganic (e.g. metal oxide) andorganic materials are present, organic layers can be arranged to besandwiched between the inorganic metal oxide films in order to providemaximum contact between the inorganic layers. Specific examples ofmultilayer barrier layers/stacks includes, for example,polycarbonate-aluminium oxide, PET-magnesium oxide, glass-tin oxide,aluminium oxide-polyacrylate-aluminium oxide, and an aluminiumoxide-silicone-aluminium oxide stack. It is thus noted that the samematerial(s) may be present in the base substrate and the barrier layer.In the context of the invention, the barrier layer is typically thestructure to be tested for its permeation properties using a sensor ofthe present invention. An example of a possible structure in which thesame material(s) is present in both base substrate and barrier layer isas follows. A sensor in which a base substrate of known permeability isused has a thin layer of low gas permeability polymer e.g.polycarbonate, incorporated into the base substrate to improve the shelflife of the sensor, for example. Subsequently, when such a sensor isused for measuring the permeation properties of a test specimen, it willbe attached directly onto a surface of the test specimen. For instance,the test can be carried out on a barrier layer, e.g. a composite liquidcrystal display (LCD) barrier stack, which may comprise the same low gaspermeability polymer that is present in the base substrate e.g.polycarbonate. In such a case, both the base substrate and the barrierlayer would have polycarbonate as a common material. In another example,the base substrate may comprise polyimide films, while the barrier layermay comprise polyimide film containing silica particles/nano-clays,normally known as intercalated or exfoliated hybrids, ornano-composites. Typical examples of a nano-clay includes any member ofthe smectite group of clay minerals, such as montmorillonites,

The electrodes of a sensor of the invention may be fabricated as wiresor strips made from metals such as copper or tin, each electrodes havingone end attached to the sensing element and the other end freelymovable. However, where the electrodes are designed as films, it isdesirable not to leave the electrodes freely movable, even though it ispossible to do so, but to immobilise them directly onto the basesubstrate or test specimen (where the barrier layer is the testspecimen). Since a film layer may be fragile and may be easily damaged,having it immobilised can help to reduce the possibility of mechanicaldamage. Accordingly, one embodiment of the invention is directed to thesensor in which the electrodes are formed on a surface of the basesubstrate.

Where the electrodes are formed on (a surface of) the base substrate,the present sensor can assume a variety of configurations. In oneembodiment, the electrodes are spaced apart to form a trench. In thiscontext, the trench is usually formed by the portion between the twoelectrodes, the edges of the trench being defined by the edge of eachelectrodes and the base of the trench being the base substrate. Thecomponents of the sensor can be arranged in several possible ways. Forexample, the sensing element may be placed over the trench, underneathit, or along the sides of the trench, as long as the sensing elementforms an electrical connection between the two electrodes.

In one embodiment, the sensing element is located in the trench. Theelectrodes can be first formed to define the structure of the trench.Subsequently, when the sensing element is deposited in the trench, it isallowed to conform to the predefined dimensions of the trench, therebyallowing the sensing element to be shaped and sized. In this manner, thetrench is analogous to being a mold or cast for the sensing element.

In this respect, depending on the required dimensions of the sensingelement, the trench may be filled either partially or fully by thesensing element. Alternatively, the sensing element may extend outsideof the trench to overlap partially the edges of the adjacent electrodes.The cross-section of the trench can typically be of a rectangular shape,square shape or any other suitable shape, as long as a suitableelectrical contact is formed between the sensing element and theelectrodes. For example, in one embodiment, if it is required to form atrench in which the base of the trench is narrower than the top, theelectrodes may be formed with ends that taper off towards the trench.When the sensing element is subsequently deposited into the trench, asensor with the configuration shown in FIG. 2C may be obtained.Alternatively, in another embodiment, if it is desired to form a sensingelement which has a wider base than the top surface, the sensing elementcan first be deposited on the base substrate, and then the electrodesare deposited to overlap partially with the edges of the sensing element(cf. FIG. 2D). In both embodiments, the sensor is designed such that theplane of contact between the sensing element and an electrode is sloped,i.e. having an angle of either more or less than 90° with respect to thesurface of the electrodes. Such a design improves the electrical contactbetween the sensing element and the electrodes, thus reducing electricalresistance at the sensor/electrodes interfaces and resulting in improvedelectrical conduction.

In another embodiment, the sensor comprises an encapsulation enclosingthe sensing element. An encapsulation can provide for a hermetical sealaround the sensor, such that the target gas is only allowed to permeatethrough the test specimen. Furthermore, an encapsulation would preventthe sensing element from coming into contact with ambient water vapourand oxygen prior to its use in tests. In addition, the encapsulationalso acts as a protective covering which buffers any physical impactthat might damage the sensing element.

The material for forming the encapsulation material, meaning theencapsulant, may comprise any type of material which is preferablysubstantially low gas permeability. Many types of polymers may be usedfor this purpose, including hydrocarbon plastics, thermoplastics,rubbers and inorganic polymers. Examples of suitable organic polymersare ultraviolet (UV) curable epoxies, polysulfides, silicone,polyurethane, polystyrene, polyalkylenes, polyimides, polybenzoxazolesand polyacrylates. If it is required to provide an encapsulation that isable to conform fully to the shape of the sensor, suitable materials arepreferably available as a mouldable gel or viscous fluid which cansubsequently be cured and hardened by heat or UV radiation.

The encapsulation can assume a variety of configurations. Onecontemplated configuration is to have the encapsulation cast as a hardcovering around the exposed surfaces of the sensor. The encapsulationmay be in direct contact with the sensing element, or it can surroundthe sensing element without being in direct contact with the sensingelement. In the former case, the sensing element is enclosed entirelywithin the encapsulation, so that the present sensor can be used tosimulate the encapsulating structure of light emitting diodes (LED). Inthe latter case, the encapsulation can provide a hollow space around thesensing element. The hollow space can be filled with an inert gas, suchas nitrogen or one of the noble gases such as argon.

In another embodiment, the sensor further comprises a cover substrate,wherein the encapsulation forms side (lateral) walls surrounding thesensing element, and the cover substrate is arranged to be in contactwith the side (lateral) walls. In more detail, in this embodiment, theencapsulation is applied onto the base substrate/test specimen aroundthe perimeter of the sensing element, thereby encircling the sensingelement, while the cover substrate is placed onto the side (lateral)wall like a lid. Such an arrangement constitutes the encapsulatingstructure housing the sensing element. Furthermore, the cover substratemay be placed on the side walls in an inert gas environment, so thatinert gas can be trapped within the encapsulating structure. In thisrespect, the encapsulant that is used can be a UV (ultraviolet) curableepoxy or any other suitable sealant. This embodiment provides a means tosimulate and thus assess the gas permeability of multi-layerorganic/inorganic thin films and encapsulating structures, such as thosefound in organic light emitting devices (OLED) and FOLEDs. The gaspermeability in turn allows an estimation of the lifespan of the oxygenor moisture sensitive device within the encapsulating structure.Essentially, the sensor is structured analogously to the encapsulatingstructure. For example, to carry out a simulation of gas permeation inthe encapsulating structure of an OLED under ambient conditions, thesame OLED device can fabricated (cf. FIGS. 7A and 7B) with the lightemitting device replaced by a sensing element of the present invention.

In general, materials that are used for forming the cover substrateshould preferably have low gas permeability in order to provide a goodhermetical seal for the sensor. The base and cover substrate cancomprise any material such as polymers, barrier coated polymer, glass,aluminium foil. Examples of materials comprising the base or coversubstrate are glass, low gas permeability polymers and metal laminatedfoils. For reasons of cost, glass, aluminium and copper are preferred insome embodiments.

As can be seen from the above, the present sensor is sufficientlyversatile to be used for the determination of gas permeability in a widevariety of applications, including the gas permeability of polymersubstrates, encapsulants, sealants, adhesives as well as overallencapsulating structures.

In one embodiment, the sensor further comprises a protective layercovering at least a portion of the sensing element. The purpose of theprotective layer is to prevent any contamination or prematuredegradation of the sensing element. This can arise due to defects in theencapsulating structure housing the sensing element. By covering exposedsurfaces of the sensing element with a protective layer, the opportunityfor any reactive gas to come into contact with the sensing element canbe reduced. If it is desired to improve the protection of the sensingelement, the protective layer can also comprise an electricallyinsulating material. For this purpose, the protective layer can beformed using any organic or inorganic material. Suitable materialsinclude metal oxides, metal fluorides, organic polymers and a mixturethereof. Examples of particularly suitable materials include, but arenot limited to metal oxides such as aluminium oxide, calcium oxide,magnesium oxide, and metal fluorides such as sodium fluoride, lithiumfluoride and magnesium fluoride. In addition to the above materials,other suitable materials include metals and metal alloys. Particularlysuitable metals and alloys include, but are not limited to copper,silver, platinum, gold and mixtures thereof. If the protective layercomprises a metal or alloy, the metal or alloy should preferably notcome into direct contact with the sensing element. This helps to ensurethat the bulk electrical properties of the sensor are not influenced bya further electrical component connected to it, thus allowing moreprecise measurements of the bulk electrical properties of thisembodiment of the sensor to be obtained. Consequently, if anelectrically conductive material such as a metal or alloy is used in theprotective layer, it may be advantageous to position an electricallyinsulating layer, comprising for instance any one of the aforesaid metaloxides or metal fluorides, between the conductive layer and the sensingelement. Accordingly, the term ‘protective layer’ may denote not only asingle layer, but it may also denote 2 or more layers, i.e. amulti-layer arrangement.

In another embodiment of the invention, the sensor further comprises anliner layer (comprising or consisting of organic and/or inorganicpolymers) interposed between the sensor and the base substrate.Materials such as inorganic coatings or layers (e.g. a metal oxidecoating) may have or develop amorphous zones or defects in the form ofpinholes, cracks, or grain boundaries. When such defects are present inthe surface of the barrier coating covering the polymer substrate, thepermeating gases can escape through the defects at a higher rate than atother locations on the surface of the sample where there are no defects.Consequently, a portion of the sensing element that is adjacent to suchdefects will be reacted at a higher rate. The non-uniform degradation ofthe sensing element may leave sections of unreacted material within thesensing element, thereby resulting in an inaccurate reading. The linerlayer usually behaves as a buffer region which sponges up (saturatewith) the permeating gases before they are desorbed homogeneously. Thehomogeneous desorption of the permeating gases results in the uniformdegradation of the sensing element, which in turn enables the decreasein electrical conductivity of the sensor to be more accuratelycorrelated to the decrease in thickness of the sensing element.

It is noted that the liner layer is not necessary, when the testmaterial comprises or consists of plain organic polymers, defect freemetals, or low gas permeability polymers having an organic top layerwhere a multi-layer barrier layer is used. In general, test materialsthat do not suffer from surface defects do not require the use of anliner layer. However, a liner layer can be applied if it is desired toimprove the performance of the sensor.

The liner layer can be deposited as a layer having thickness rangingfrom 10 nm up to a few microns or higher. It can comprise any organicmaterial that exhibits relatively little gas barrier properties may beused. Examples of suitable materials include organic polymers such aspolyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, as well ascopolymers thereof, Other suitable polymers include parylene typepolymers, and acrylic polymers such as polyacrylates (e.g. poly methylmethacrylate), polyacrylic acids, and polyacrylamides. Cellophane filmsare also suitable for use in or as an organic liner layer. Furthermore,the liner layer can also comprise a combination of inorganic polymerssuch as silicone-type polymers, polysilanes, polygermanes, polystannanesand polyphosphazenes. It is noted in this conjunction that some of thematerials specified for the base substrate (layer) and barrier layer arealso common to the liner layer, meaning that it is possible that thesame material is selected for the base substrate, barrier layer and/orliner layer. For example, if the base substrate comprises a particularpolymer, the liner layer can also comprise the same polymer. In anotherexample, if the base substrate is a polymer or a composite comprisinginorganic materials such as silicon oxide or other oxides includingnano-clays or particles, the barrier layer also may likewise comprisethe same polymer or composite materials.

In another aspect, the invention relates to a method of producing asensor for measuring the gas permeability of a test material asdescribed above, said method comprising providing two electrodes, andconnecting an electrically conductive sensing element that comprises awater and/or oxygen sensitive material to said two electrodes.

Both the sensing element and electrodes can be obtained as prefabricateditems with a specific form e.g. a wafers, strips or desiccant pads,which can be assembled directly to form the sensor. Alternatively, ifthe sensing is to be formed as a film on the surface of a test specimenvia film deposition methods, e.g. thermal evaporation or sputtering orany surface technology, the sensing element can be procured in a formthat is suitable for use in film deposition equipment and thensubsequently shaped and molded into a desired form to form the sensor.

For the purpose of carrying out the present method, the electrodes canbe provided for fabrication of the sensor in several ways. For example,it is possible to first suspend the electrodes on a pair of holders andthen solder the sensing element to the electrodes. Alternatively, in oneembodiment, the sensing element is immobilised on a support, e.g. on asurface of a base substrate, and thereafter the electrodes are solderedto the sensing element. The skilled person will appreciate that it isnot always necessary to solder the sensing element and the electrodes toform a functional electrical connection. It may be sufficient to formthe sensing element directly onto the electrodes, or vice versa, and tosecure the arrangement by means of a weight placed thereupon, withouthaving to connect them via soldering, conductive adhesives or otherconnection means. However, depending on the application and the type ofelectrode and sensor utilised, any suitable type of connection means maybe used.

In another embodiment of the present method, the electrodes are formedon a surface of a base substrate (which can be the material to betested). Where the electrodes are electrical wires, the electrodes mayfirst be immobilised on the substrate with the contact areas exposed,leaving a gap between the electrodes that corresponds to the location ofthe sensing element, and then forming the sensing element in the gapcontacting the electrodes. Where the electrodes are to be formed as thinor thick films, conventional film deposition techniques such as vacuumvapour deposition (VVD), physical vapour deposition (PVD), chemicalvapour deposition (CVD), thermal evaporation, sputtering, or any othersurface deposition technology may be used. The material for the sensingelement may then be evaporated into the gap formed between theelectrodes using a suitable mask, or conventional lithography or etchingtechniques, or any thermal evaporation techniques such as thosedescribed for forming the electrodes.

The present method can be carried out according to any suitable sequenceof steps. For example, the electrodes can first be formed on the testmaterial before depositing the sensing element. Alternatively, it isalso possible to first form the sensing element onto the test materialbefore forming the connecting electrodes.

When measurements requiring high levels of sensitivity (e.g. lower than10⁻³ g/m²/day) are to be carried out with a sensor of the invention,tests on low gas permeability materials should ideally be performed withtest materials and sensor components that are substantially devoid ofcontaminants, such as oxygen and water reactive substances or macroscale adsorbed particles found on their surfaces. For this reason, it isdesirable (though not necessary) that the sensor components as well asthe test material are cleaned to remove any contaminant, including macroscale adsorbed particles, which may be introduced during the course ofmanufacturing, or when carrying out the deposition processes. Anycombination of conventional surface cleaning techniques commonly used inthe semiconductor industry may be used, such as laser cleaning, physicaland chemical plasma processes, UV radiation and silicon flux.

In one embodiment of the method of producing a sensor of invention, thesurface of the test material is subjected to a surface preparationprocedure in order to remove such contaminants. One preferred surfacepreparation procedure comprises rinsing the test material or substrateand its deposited electrode with an alcohol, blow-drying with an inertgas, and vacuum degassing. Suitable alcohols which may be used includessecondary, tertiary, and branched-chain alcohols, e.g. iso-propylalcohol or iso-butyl alcohol. In practice, the skilled person willappreciate that cleaning with chemicals such as acetone and/orshort-chain primary alcohols such as methanol or any other chemical thatcan act as an organic solvent towards any polymer in the substrate, maynot be suitable in certain embodiments in the method of preparing asensor of the invention. Accordingly, if a polymer that is not resistantto such chemicals is present in the base substrate and/or liner layer,these chemicals may not be used in the cleaning procedure. Nevertheless,if all polymers or other materials present in the base substrate areresistant to these chemicals, they may then be used for cleaning.

In the above surface preparation procedure, after the sensor is rinsedwith alcohol, it is blow-dried using high pressure gas to get rid oftraces of the rinsing alcohol. Subsequently, the sensor is placed in avacuum oven in order to ensure that the surface is free of adsorbedmoisture or oxygen. In cases where organic polymers are used in thesensor, the temperature at which vacuum degassing may be carried out maythen depend on the organic polymer used and the respective glasstransition temperature(s). In general, suitable temperatures forcarrying out vacuum degassing are below the glass transition temperature(T_(g)) of the polymer(s) present in the base substrate. For example,where conventional LCD barrier stacks are used, vacuum degassing may becarried out at temperatures of between 30° C. to 100° C., preferablybetween 50° C. to 85° C., and under partial pressure for 1 to 80 hours,preferably 6 to 60 hours.

In a further embodiment, the surface on which the sensor is formed istreated with argon gas plasma after vacuum degassing. Prior to formingthe sensing element, RF argon plasma may be used to bombard the surfaceof the barrier film with low energy ions in order to remove traceamounts of surface contaminants. Depending on the surface condition,plasma treatment can be carried out using plasma power of 30 W to 2 kW,using argon gas flow of 20 sccm to 100 sccm and a duration of 10 s to 2hr. For example, when indium tin oxide (ITO) is used as a barrier layer,a suitable plasma treatment comprises RF power of 200 W, substrate bias50V, argon gas of 70 sccm and treatment duration of 5 to 8 eightminutes.

In order to decrease the chances of any unwanted degradation of thesensing element, the exposed surfaces of the sensing element can beprotected from exposure to water vapour or oxygen by applying aprotective and preferably insulating layer thereon, after the depositionof the sensing element has been carried out. The insulating layer cancomprise any type of organic or inorganic layer, such as a thin filmdeposited by conventional processes e.g. PVD or CVD. Transparent filmssuch as lithium fluoride and magnesium fluoride are suitable for thispurpose.

After carrying out the plasma treatment process, a liner or buffer layercan be formed if the test substrate comprises an inorganic (e.g. metalor metal oxide) layer as a terminating or top layer. The liner layer canbe deposited as a thin layer by spin coating or vacuum evaporation orany other conventional surface deposition process.

Another embodiment of the present method further comprises encapsulatingthe sensing element. Conventional encapsulation techniques forencapsulating the sensing element may be used and may be carried out inan inert environment, such as a glove box. For example, transfermolding, which involves placing the sensing element into a cavity of amold and then injecting a thermosetting material into the cavity, may becarried out in order to form the sensor. The thermosetting material isallowed to flow over the sensing element until it is covered entirely,and is thereafter cured so that it hardens into a protective covering.When the sensor includes a substrate, the sensing element and at least aportion of the substrate may be placed into the mold cavity. Thethermosetting material can be made to flow over the whole sensingelement on one side of the substrate, and then allowed to set. Anultraviolet (UV) curable sealant, e.g. epoxy, can be used for thispurpose.

One variation to the above encapsulation procedure comprises applyingthe sealant to the rim of the substrate, followed by sealing the sensingelement with a cover substrate, e.g. a glass cover. The cover substratecan be transparent in order to provide a view of the sensing elementduring the course of carrying out the performance test. However, anopaque cover may be used if it is not necessary to view or to takephotographs of the sensing element as it degenerates during the courseof the measurement test. As an added measure to maintain a contaminantfree environment in the test chamber, the cover substrate may also becleaned before use, for example, by rinsing it in acetone and water.

In a further aspect, the invention is also directed to a system formeasuring gas permeability of a test material, said system comprisingthe sensor of the present invention. The system may comprise a testchamber, a constant current source, a digital signal analyser and ameter for measuring resistance.

In more detail, the test sample with its accompanying sensor may placedin a suitable chamber where the humidity, pressure and temperature canbe set to any desired level in order to simulate a particularenvironment. If a humidity chamber is used for the test, the permeationof other gases, e.g. oxygen, through the test material may also bestudied by introducing these gases into the chamber during the test. Theelectrodes of the sensor are connected to a constant current sourcemeter which may in turn be interfaced with a computer that records thedata transmitted from the meter at a regular interval. Any suitableconstant current source meter can be used in the present system.Examples are those which are available from Keithley (Ohio, USA), BridgeTechnology (Arizona, USA), or Glen Spectra (Middlesex, UK). A dataplotting software may be used so that the resistance of the sensor canbe plotted as a function of time.

The system further comprises a dynamic spectrum analyser to evaluate the1/f noise of the sensor when carrying out the test. Suitable spectrumsignal analysers which may be used include HP35670A, HP3561A or HP35665Afrom Agilent (California, USA) and SR785 or SR780 from Stanford Research(CA, USA). It is also desirable to choose an analyser that provides afast Fourier transform (FFT) routine to evaluate spectral density of thenoise signal. Optionally, a low noise amplifier, such as StanfordResearch's SR570 low noise amplifier, can be used to amplify the noisesignals prior to being processed by the signal analyser.

The measuring system may incorporate not only a humidity chamber butalso other suitable types of test chambers in which test materials canbe tested against other gases apart from water vapour and underdifferent conditions. Examples of suitable chambers include pressurisedgas chambers and hyperbaric oxygen chambers. A preferred setup of a testchamber in which gas permeation measurements can be carried out (seeFIG. 13) may comprise a hollow centre portion that is partitioned by atest specimen (substrate or film) into two sections. One section isprovided with a test gas at a pre-determined pressure and temperatureand on the other side of the test specimen, an inert gas is provided atthe same or different pressure as a sweep gas to evacuate any test gaspresent. Prior to mounting the test specimen into the gas chamber, asensor is fabricated on the test specimen and attached to the side wallsof the gas chamber.

Another aspect of the present invention is directed a method fordetermining gas permeability. This method comprises contacting thesensing element of a sensor of the present invention with water and/oroxygen during a measurement test, measuring the change in electricalconductivity of the sensor over a period of time, and calculating thegas permeability of the test material based on the measurements.

The calculation of gas permeability is based on the rate at which thesensing element reacts with the target gas. One initial value that isusually established in order to calculate the rate of reaction is theinitial quantity of sensing element. For this reason, a specifiedquantity of material can advantageously be used for the fabrication ofthe sensing element. Alternatively, the sensing element can be designedto have a pre-determined dimension (length×breadth×height), from whichthe quantity of sensing element can be calculated using the formula (I):

${{{Amount}\mspace{14mu}{of}\mspace{14mu}{sensing}\mspace{14mu}{element}\mspace{14mu}{reacted}\mspace{11mu}({mol})} = \frac{{Volume} \times {Density}}{A_{r}}},$wherein the variable “Volume” refers to the volume of the sensingelement, “Density” refers to the density of the sensing element, and“A_(r)” refers to the relative atomic mass of the sensing element. Bymultiplying the (molar) quantity of sensing element used in the sensorby the stoichiometric amount of reacting of water or oxygen or any othergas, the quantity of gas which reacts with the sensing element can beobtained. This quantity is taken to be equivalent to the quantity of gastransmitted through the test specimen or test structure.

The amount of time taken for the sensor to cease conducting electricitypartially or fully, i.e. the measured electrical conductivity=0 Mho, isalso usually needed, and can be determined with a source meter. Gastransmission rate can be calculated from the formula (II):Gas transmission rate=Quantity of water transmitted per unit surfacearea (g)×(24 hr/Time)where the variable “Time” refers to the time taken for the sensor tocease conducting. In this respect, it is noted that it may be useful tocalibrate the sensor by determining the time needed for the entireconsumption/reaction of a specific amount/configuration of sensingelement. Once such a calibration is done, a reference point is obtainedwhich the subsequent use of a sensor with a sensing element having thesame dimensions and characteristics can rely upon. In such a subsequentuse, the calibrated sensor may then require only a partial reaction ofthe sensing element.

Instead of calculating gas transmission based on the reacted amount ofsensing element, it is also possible to calculate gas transmission basedon other variables such as electrical resistance or conductivity of thesensing element, for instance. In one embodiment, the measurement of 1/fnoise of the sensor is carried out to determine the gas permeability. Abrief introduction to 1/f noise in the context of this invention is asfollows. It is known that signals such as a current flowing through aresistor, the resistance of the resistor, or the voltage across theresistor, exhibit random fluctuations. These fluctuations, termed thenoise of the signal, are characteristic of the signal. The power spectraof the noise P(f) as a function of the frequency f generally behavesaccording to the equation: P(f)=1/f^(β). Where β=1 or β is close to 1,the type of noise exhibited is normally referred to simply as 1/f noise(or pink noise) which occurs very often in processes found in nature(for example the 1/f noise is seen in any molecular movement or electronmovement). If β=0, it is normally called white noise. If β=2, it iscalled brown noise. The power spectra P(f) of all noise forms such as1/f noise, white noise, brown noise and other type of noise can all bemeasured with a dynamic spectrum analyser. In determining the 1/f noiseof a signal as a function of another measurable variable that changesduring the course of the measurement test, such as electricalresistance, the following calculation method may be taken. Firstly, thefluctuation of the signal with respect to a mean value over a period oftime is recorded as fluctuation data. This mean value can for instancebe the average value of the signal over that period of time.Subsequently, the fluctuation data is Fourier-transformed from the timedomain into the frequency domain. Finally, the probability offluctuation (also known as the spectral density of the signal) isplotted as a function of the other variable, such as electricalresistance as mentioned above. Such a plot typically shows that thechange in spectral density is proportional to the change in thevariable. In this way, measurements of 1/f noise can be directlycorrelated to changes in the variable.

This embodiment relies on the relationship between the change in 1/fnoise (dN) and the change in resistance of the sensor (dR). In order todetermine this relationship, a digital signal analyser can be useddirectly to evaluate 1/f noise as a function of sensor resistance as thetest progresses. The measurement of resistance of the sensor issimultaneously carried out with the measurement of 1/f noise during thetest. A computer interfaced with the system can be programmed totabulate and graph N vs R, and thus obtain the rate of change of 1/fnoise with respect to resistance (dN/dR).

Experimental variables and initial values that are usually establishedfor the calculation of gas permeability in the embodiment using the 1/fnoise include the initial thickness of the sensing element, initialsensor resistance, conductance, time taken for the test, and thesensitivity of the digital signal analyser's 1/f noise analysis (S).

The following general formulas show the calculations that need to becarried out to obtain gas transmission rate based on (dN/dR):Change in resistance per unit change in 1/f noise (R _(1/f))=S÷(dN/dR)  (III)Change in sensor thickness=R _(1/f)×(sensing element thickness÷initialsensor resistance)   (IV)

Once the change in sensor thickness has been determined, gastransmission rate can be readily calculated using Formula (I) and (II).

One advantage in monitoring the change in 1/f noise instead of measuringdirectly the change in the variable to be measured can be attributed tothe high sensitivity level of sensitivity provided by digital signalanalysers in reading 1/f noise (typically less than 1×10⁻¹⁴ V²_(rms)/Hz). This level of sensitivity can be about 5 to 7 orders ofmagnitude smaller than the value of the 1/f noise that is beingmeasured. This facilitates the detection of very fine oxidation in thesensor and therefore enables the measurement of gas transmission ratesof less than 10⁻⁸ g/m²/day. For example, the change in 1/f noise of thesensor can be measured and plotted as a function of sensor resistance.From the plotted graph, it will be seen that dN/dR is very small,meaning that the measurable unit change in 1/f noise corresponds to verysmall unit change of about 10⁻⁷ to 10⁻⁸ ohms in resistance. Thistranslates into a sensitivity for detecting water vapour transmissionrate of about 10⁻⁸ g/m²/day and oxygen transport rates of 10⁻⁸ cc/m²/dayat temperatures of between 20° C. to 95° C. This level of sensitivity issufficient for the purposes of testing low gas permeability materialsand structures such as that found in FOLEDs.

In addition to improved sensitivity, the present invention has theadvantage in being able to determine diffusion coefficient, permeabilitycoefficient as well as the solubility coefficient of a test materialusing a single test, whereas previously it was only possible todetermine ach of these coefficients by carrying out separate independenttests.

From the foregoing description, it can be seen that the invention can beapplied to many types of applications including, for example, thetesting of flexible and rigid polymer substrates having either single ormultiple protective layers, encapsulation structures with or withoutcover substrates, epoxy or adhesive materials or taps on the rim orpolymer substrate, substrates with barrier/protective layer comprisingmulti-layer organic or inorganic thin films. This multi-layer can haveseveral ceramic oxide layers or inorganic layers and organic layers toprovide low gas permeability performance encapsulation. Suchapplications are widely employed in electronics packaging applications,such as display panels using hermetically sealed OLED devices, liquidcrystal displays (LCD) and integrated chip packages/structures. Thematerial which are used for forming these substrates include polymerssuch as polyethylene, polyethylene sulphide, polycarbonate, substrateswhich can be laminated with a single or multiple layers of metal oxideor ceramic barrier coatings, as well as glass substrates. Otherapplications in which the present invention may be used are themeasurement of gas permeation properties of LEDs, OLED, and LCDencapsulation, hard disk drive metallic enclosures, as well as food anddrug packages, vacuum applications, ammunition containers, and plasticcontainers.

The invention will be further explained by the following non-limitingexamples and the accompanying figures, in which:

FIG. 1 shows a sectional view of one embodiment of a sensor of thepresent invention.

FIGS. 2A to 2F show different embodiments of the sensor supported by abase substrate. FIG. 2G shows a sensor that is supported on a barriercoated substrate.

FIGS. 3A and 3B show different embodiments of a sensor of the inventionincorporating a liner layer.

FIGS. 4A and 4B show a sensor that incorporates an encapsulation.

FIG. 5 shows a sensor that is formed on a base substrate and whichcomprises an encapsulation.

FIG. 6 shows a sensor that is configured in an analogous manner to aconventional OLED device.

FIGS. 7A and 7B show two different types of OLED encapsulationstructures.

FIG. 8 depicts an encapsulated sensor that is formed on a substrate andwhich incorporates a protective layer formed on a surface of the sensingelement.

FIG. 9 depicts a schematic diagram of a measurement system.

FIGS. 10A to 10C show the measurements obtained in a test wherecommercial substrates were coated with silicon dioxide coating. FIG. 10Ashows the graph of ‘Calcium Resistance vs. Time’; FIG. 10B shows thegraph of ‘Calcium Conductance vs. Time’; FIG. 10C shows a graph of ‘1/fnoise of the sensor vs. Resistance of the sensor’.

FIGS. 11A and 11B are digital images of a calcium sensing element usedin a test on a polyethylene terephthalate (PET) substrate. FIG. 11Ashows the images of the calcium sensing element taken at different timeintervals at 10× magnification. FIG. 11B shows the image of the calciumsensing element in a control setup.

FIGS. 12A and 12B are digital images of a calcium sensing element usedin a test on a glass substrate. FIG. 12A shows the degradation patternof the sensor at different time intervals. FIG. 12B shows thereflectance of the calcium sensing element at different time intervals.

FIG. 13 shows a gas permeation test cell which can be used to carry outpermeation tests with a sensor of the invention.

EXAMPLE 1 Exemplary Embodiments of a Sensor of the Invention

Exemplary Embodiment 1

FIG. 1 depicts a sensor 10 according to the invention, in which asensing element 100, such as a strip of calcium, is connected to a pairof electrodes 102. It can be seen from the figure that the electrodes102 are connected to the short edge of the sensor. A contact adhesivesuch as bond pads may be applied over the short edge of the sensingelement used to improve the contact between the electrode and thesensing element. The electrodes may be connected either at the shortedge or the long edge of the sensing element.

Exemplary Embodiment 2

FIGS. 2A to 2G shows various embodiments of a sensor 200 that issupported by a base substrate 104. In FIG. 2A, the electrodes 102 areheld above the base substrate 104.

FIGS. 2B to 2G show a sensor 200 in which the electrodes 106 are locatedon a surface of the test material and are spaced apart to form agap/trench in between. The trench is occupied by the sensing element100. In FIG. 2B, the plane of contact between the electrodes and thesensing element is vertical with respect to the base substrate. In FIG.2C, the plane of contact is not vertical to the surface of the substratebut forms an angle that is less than 90°. The cross-sectional view ofthe sensor shows a sensing element which has a narrow base surface and awide top surface. This configuration can for example be formed by firstforming the electrodes on a substrate, and then controlling thedeposition process to form electrodes which taper off at the ends.Subsequently, the sensing element 100 is deposited between the twoelectrodes.

FIG. 2D shows a sensor 200 where the sensing element 100 has a basesurface which is wider than the top surface. In this example, the angleof contact between the sensing element and the electrodes is about 45°.FIG. 2E shows a sensor 200 in which the sensing element extends out ofthe trench between the electrodes 106 and overlaps partially with theelectrodes 106. FIG. 2F shows a sensor 200 in which the sensing elementextends over the surface of the substrate 104 and the electrodes 106 areformed on the sensing element. FIG. 2G shows a sensor 200 in which thesubstrate 104 is coated with a barrier layer 108.

Exemplary Embodiment 3

FIGS. 3A and 3B depicts embodiments of a sensor 300 which incorporatesan liner layer 110 between the sensing element and the base substrate.FIG. 3A shows the liner layer to be positioned between the basesubstrate and the sensing element. FIG. 3B shows the liner layer to beflush with the electrodes, i.e. having about the same thickness as theelectrodes.

Exemplary Embodiment 4

FIG. 4A shows a sensor 400 which is enclosed in an encapsulation 112.FIG. 4B shows a sensor in which the encapsulation 112 is not in directphysical contact with the sensing element 100 but provides a hollowspace 114. The hollow space can be evacuated or filled with an inert gasif required. In one specific embodiment, the encapsulation 112 is anepoxy resin and hollow space 114 is filled with argon gas. In anotherspecific embodiment, the encapsulation 112 is a polyurethane resin andthe hollow space is filled with nitrogen gas.

Exemplary Embodiment 5

FIG. 5 shows an encapsulated sensor 500 in which the electrodes 106 areformed on a surface of a substrate 104. The electrodes 106 are spacedapart to form a trench which accommodates the sensing element 100. Thethickness of the sensing element 100 is approximately level with thethickness of the electrodes 106. An encapsulation 112 is formed over thesensing element 100.

Exemplary Embodiment 6

FIG. 6 shows a sensor 600 which incorporates an encapsulation andcomprises a cover substrate 114 adjacent to the encapsulation. In thisembodiment, a layer of encapsulation 112 is applied on the electrodesand base substrate, around the sensing element 100. The cover substrate114 is placed on the encapsulation 112, thereby sealing the sensingelement 100. The hollow space 116 enclosed within the encapsulation andcover substrate may be filled with an inert gas such as argon ornitrogen. This configuration is suitable for testing gas permeationproperties of OLED packages, which have structures shown in FIGS. 7A and7B.

Exemplary Embodiment 7

FIG. 8 shows a sensor 800 which incorporates an encapsulation/sealant112 comprising epoxy and a glass cover substrate 114. In thisembodiment, a layer of encapsulation 112 (epoxy sealant) is applied onthe test specimen 104 and the pair of metal tracks 106 constituting theelectrodes. Calcium is used as sensing element 100 and the top of thesensing element 100 surface is covered with a protective layer 118. Theglass cover substrate is placed on the encapsulation, thereby sealingthe sensing element. The hollow space 116 enclosed above the sensingelement is filled with nitrogen.

EXAMPLE 2 Fabrication of a Gas Permeability Sensor

In this example, a sensor as illustrated in FIG. 8 and as shownschematically in FIG. 9 was produced. The test sample (thickness 175 μm)comprised a 133 μm thick polycarbonate film coated with 30 nm thicksilicon oxide barrier layer. The test sample (substrate) was slit into50 mm by 50 mm dimensions with a pneumatically operated hollow diepunch-cutting machine. After slitting was performed, silver wasdeposited onto the test sample using a suitable mask to form twoelectrically conductive tracks onto the test sample surface. Theconductive tracks were orientated such that the width of each trackfaces each other. Two strips of conductive metal (silver) tracks with athickness of 300 nm (18 mm length and 20 mm width) were deposited on thetest sample, thereby forming in between them a 10 mm length×20 mmwidth×300-μm height gap in which a calcium sensing element wassubsequently formed as explained below. Measurements as explained inExample 3 were taken, assuming a normalised sensing element area of 1m².

After the conductive track was fabricated, the test sample were rinsedwith isopropyl alcohol (IPA) and blow-dried with nitrogen. Theblow-dried substrates were then placed in a vacuum oven for degassingany absorbed moisture or oxygen. The pressure in the oven was set to10⁻¹ mbar and vacuum degassing was carried out for several hours at anelevated temperature of between 60-100° C., which is below the glasstransition temperature of the polymer present in the barrier stack usedin this example. The vacuum oven used here was additionally equippedwith fore line traps to prevent the backflow of hydrocarbon oil from thevacuum pump to the vacuum oven.

Immediately after the degassing process, the partially formed sensor andtest sample were transferred to a ULVAC SOLCIET Cluster Tool whereplasma treatment was carried out for several hours. Radio frequency (RF)argon plasma was used to bombard the surface of the barrier film withlow energy ions in order to remove surface contamination. The basepressure in the chamber was maintained below 4×10⁻⁶ mbar. The argon flowrate was at 70 sccm and RF power was set at 200 W.

After the plasma treatment process, a liner layer comprising an acrylicpolymer was deposited as a thin layer of about 100 nm by spin coating.The liner layer presents relatively little gas barrier properties, butensures uniform layer-by-layer oxidation through the calcium sensingelement.

After the deposition of the thin organic buffer layer, the test sampleswere transferred to a vacuum evaporation chamber. Subsequently, calciumwas deposited through a suitable mask into the space between the twoelectrodes to form a 150 nm thick layer. After calcium deposition, a 100nm insulating film comprising LiF was deposited by PVD onto the calcium.

Subsequently, the partially formed sensor was transferred to a glove boxwhere it was encapsulated. UV curable epoxy was applied onto the rim ofthe test sample. A glass slide of matching dimension was placed onto theepoxy to seal off the calcium sensing element. A 400 W metal halidelight source (2000-EC series UV light source, DYMAX Corporation) wasused to cure the epoxy for about two minutes. The wavelengths of thelight source was between 300 nm and 365 nm, with the respectiveintensities of 22 mW/cm² and 85 mW/cm². The entire encapsulation processwas undertaken under inert nitrogen gas atmosphere in order to ensureoptimum encapsulation.

EXAMPLE 3 Water Vapour Permeation Test Using 1/f Noise Measurement

The encapsulated sensor fabricated in Example 2 (cf. FIG. 9) whichcomprise the test sample comprising the barrier stack with silicondioxide was transferred into a humidity chamber (WK1 Model, Weiss,Germany). A HP35670A digital signal analyser was connected to theelectrodes to monitor the 1/f noise of the sensor. At the same time, aKeithley 24203A source meter was connected in parallel to the electrodes(cf. FIG. 9) to provide a constant current source to the sensor and tomonitor the rate of change of calcium resistance over time. 1/f noise,calcium conductance and resistance measurements were done at atemperature of 40° C., 90% relative humidity and atmospheric pressure.

The graph of change in calcium resistance against time is shown in FIG.10A. As can be seen from the figure, a lag time of about 2-3 hours isobserved, during which the calcium sensor shows no degradation. This lagtime represents the duration required for the water vapour to traversethe test sample before reaching the sensing element.

The graph of rate of change of calcium conductance against time as shownin FIG. 10B was also obtained. As can be seen from the figure, the timetaken for calcium conductance to fall from the initial conductance isabout 2.8 hours from the start of the test. Conductance decreases tozero in about 7 hours, meaning that about 4.2 hours was needed for thecalcium sensing element to be completely consumed.

1/f noise measurements made by the digital signal analyser was plottedas a function of sensor resistance. The graph is shown in FIG. 10C.

Based on the data, the quantitative evaluation of water vapourtransmission properties could be carried out as shown in the furtherexamples below.

EXAMPLE 4 Calculation of Water Vapour Transmission Rate—Sensitivity ofNoise Measurements

This example describes a procedure for deriving water vapourtransmission rate through a conventional barrier stack such as the oneused in the above Example which is coated with silicon dioxide coatingbased on sensitivity of noise measurements according to the invention.

The data needed for carrying out the present calculations is taken fromthe resistance and noise measurements from Example 1. The time taken tooxidise 150 nm of calcium was about 4.2 hours as shown in the graph ofFIG. 10B.

The 1/f noise analysis sensitivity of the digital signal analyzer(HP35670A) was specified to be less than 10⁻¹⁴ V² _(rms)/Hz at 64 Hz & 1mA constant current.

Using the data obtained from the measurements carried out in Example 1,the derivation of water vapour transmission rate was carried out asfollows:

$\begin{matrix}\left. 1 \right) & {{Rate}\mspace{14mu}{of}\mspace{14mu}{change}\mspace{11mu}{of}\mspace{11mu}{1/f}\mspace{14mu}{noise}\mspace{14mu}(N)\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{sensor}\mspace{14mu}{with}\mspace{14mu}{respect}} \\\; & {{{to}\mspace{14mu}{change}\mspace{14mu}{of}\mspace{14mu}{resistance}\mspace{14mu}(R)\mspace{14mu}{i.e.\mspace{11mu}\left( {{\mathbb{d}N}/{\mathbb{d}R}} \right)}} =} \\\; & {{{Slope}\mspace{14mu}{of}\mspace{14mu}{graph}\mspace{14mu}{of}\mspace{14mu}{1/f}\mspace{14mu}{noise}\mspace{14mu}{vs}\mspace{14mu} R\mspace{14mu}\left( {{Figure}\mspace{14mu} 10C} \right)} =} \\\; & {3.38 \times 10^{- 7}{V_{rms}^{2}/{Hz}}\mspace{11mu}\left( {{at}\mspace{14mu}{the}\mspace{14mu}{value}\mspace{14mu}{of}} \right.} \\\; & \left. {R = {23.13\mspace{11mu}{Ohms}}} \right)\end{matrix}$

-   -   Hence, for the change in resistance of 1Ω, there is a change of        3.38×10⁻⁷ V² _(rms)/Hz in the 1/f noise.    -   2) Conversely, for a change of 1×10⁻¹⁴ V² _(rms)/Hz in 1/f        noise, the change in sensor resistance=1×10⁻¹⁴÷3.38×10⁻⁷=2×10⁻⁸        ohms.    -   3) For the change of 2×10⁻⁸ ohms, the change in calcium        thickness=0.8×10⁻⁵ nm.

$\begin{matrix}\left. 4 \right) & {{{Corresponding}\mspace{14mu}{change}\mspace{14mu}{in}\mspace{14mu}{amount}\mspace{14mu}{of}\mspace{14mu}{Calcium}\mspace{11mu}({mol})} =} \\\; & {\frac{\text{Volume} \times \text{Density}}{A_{r}\left( \text{Ca} \right)} =} \\\; & {\frac{0.8 \times 10^{- 14}m^{3} \times 1.55\mspace{11mu}\text{g/}10^{- 6}m^{3}}{40.08\mspace{14mu} g\text{/}{mol}} =} \\\; & {3 \times 10^{- 10}\mspace{11mu}{mol}} \\\; & \; \\\left. 5 \right) & {{Two}\mspace{14mu}{water}\mspace{14mu}{molecules}\mspace{14mu}{are}\mspace{14mu}{required}\mspace{14mu}{to}\mspace{14mu}{react}\mspace{14mu}{with}\mspace{14mu}{one}} \\\; & {{{calcium}\mspace{14mu}{{atom}.\mspace{11mu}{Hence}}},{{the}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{water}}} \\\; & {{{molecules}\mspace{14mu}{required}\mspace{14mu}{to}\mspace{14mu}{react}\mspace{14mu}{with}\mspace{14mu} 3 \times 10^{- 10}\mspace{11mu}{mol}\mspace{14mu}{of}\mspace{14mu}{Calcium}} =} \\\; & {{2 \times 3 \times 10^{- 10}\mspace{11mu}{mol}} =} \\\; & {6 \times 10^{- 10}\mspace{11mu}{mol}} \\\; & \; \\\left. 6 \right) & {{The}\mspace{14mu}{molecular}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu} H_{2}O\mspace{14mu}{is}\mspace{14mu} 18\mspace{14mu} g\text{/}{{mol}.}} \\\; & {{Hence},{{{the}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu} 2.2 \times 10^{- 10}\mspace{11mu}{mol}\mspace{14mu}{of}\mspace{14mu} H_{2}O} =}} \\\; & {{6 \times 10^{- 10}\mspace{11mu}{mol} \times 18\mspace{11mu}\text{g/}{mol}} \approx} \\\; & {1 \times 10^{- 8}\mspace{11mu}\text{g/}m^{2}} \\\; & {{Therefore},{{the}\mspace{14mu}{sensitivity}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{digital}\mspace{14mu}{signal}\mspace{14mu}{analyser}}} \\\; & {{for}\mspace{14mu}{the}\mspace{14mu}{detection}\mspace{14mu}{of}\mspace{20mu}{water}\mspace{14mu}{vapour}\mspace{14mu}{permeation}\mspace{14mu}{is}} \\\; & \underset{\_}{1 \times 10^{- 8}\mspace{11mu}\text{g/}{m^{2}.}} \\\; & \; \\\left. 7 \right) & {{{Water}\mspace{14mu}{vapour}\mspace{14mu}{transmission}\mspace{14mu}{rate}\mspace{14mu}({WVTR})} =} \\\; & {{1 \times 10^{- 8}\mspace{11mu}\text{g/}m^{2} \times \left( {24\mspace{14mu}{hrs}{\text{/day} \div 4.2}\mspace{11mu}{hrs}} \right)} \approx} \\\; & {5.71 \times 10^{- 8}\mspace{11mu}\text{g/}m^{2}\text{/day}}\end{matrix}$

From the above, it can be seen that the sensitivity of noise measurementmethod is able to measure a water vapour transmission rate of less than10⁻⁶ g/m²/day and is therefore sufficiently sensitive for themeasurement of low gas transmission rates.

EXAMPLE 5 Calculation of Water Vapour Transmission Rate—DirectMeasurement of Calcium Resistance

The present example describes a method of deriving water vapourtransmission rate based on direct measurements of calcium electricalresistance.

The data needed for carrying out the present calculations is taken fromthe resistance and noise measurements from Example 2. The time taken tooxidise 150 nm of calcium was about 4.2 hours as shown in the graphs ofFIGS. 10A and 10B.

The derivation of water vapour transmission rate was carried out asfollows:

$\begin{matrix}\left. 1 \right) & {{{Amount}\mspace{14mu}{of}\mspace{14mu}{Calcium}\mspace{11mu}({mol})\mspace{14mu}{reacted}\mspace{14mu}{when}}\mspace{14mu}} \\\; & {{{conductance}\mspace{14mu}{falls}\mspace{14mu}{to}\mspace{14mu} 0\mspace{14mu}{Mho}},\left( {i.e.\mspace{11mu}{calcium}}\mspace{14mu} \right.} \\\; & {\left. {{sensor}\mspace{14mu}{is}\mspace{14mu}{completely}\mspace{14mu}{reacted}} \right) =} \\\; & {\frac{{Volume} \times {Density}}{A_{r}({Ca})} =} \\\; & {\frac{150 \times 10^{- 9}m^{3} \times 1.55\mspace{11mu}{\text{g}/10^{- 6}}m^{3}}{{A_{r}({Calcium})}\mspace{11mu}\text{g/}{mol}} =} \\\; & {5.8 \times 10^{- 3}\mspace{11mu}{mol}} \\\; & \; \\\left. 2 \right) & {{Two}\mspace{14mu}{water}\mspace{14mu}{molecules}\mspace{14mu}{are}\mspace{14mu}{required}\mspace{14mu}{to}\mspace{11mu}{react}} \\\; & {{{with}\mspace{14mu}{one}\mspace{14mu}{calcium}\mspace{14mu}{{atom}.\mspace{11mu}{Hence}}},{{the}\mspace{14mu}{number}}} \\\; & {{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{water}\mspace{14mu}{molecules}\mspace{14mu}{required}} \\\; & {{{to}\mspace{14mu}{react}\mspace{14mu}{with}\mspace{14mu} 5.8 \times 10^{- 3}\mspace{11mu}{mol}\mspace{14mu}{of}\mspace{14mu}{calcium}} =} \\\; & {{2 \times 5.8 \times 10^{- 3}\mspace{11mu}{mol}} =} \\\; & {11.6 \times 10^{- 3}\mspace{11mu}{mol}} \\\; & \; \\\left. 6 \right) & {{The}\mspace{14mu}{molecular}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu} H_{2}O\mspace{14mu}{is}\mspace{14mu} 18\mspace{14mu}\text{g/}{{mol}.}} \\\; & {{Hence},{{{the}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu} 11.6 \times 10^{- 3}\mspace{11mu}{mol}\mspace{14mu}{of}\mspace{14mu} H_{2}O} =}} \\\; & {{11.6 \times 10^{- 3}\mspace{11mu}{mol} \times 18\mspace{14mu}\text{g/}{mol}} =} \\\; & {{0.208\mspace{11mu}\text{g/}m^{2}} \approx} \\\; & {0.2\mspace{11mu}\text{g/}m^{2}} \\\; & \; \\\; & {{Therefore},{{the}\mspace{14mu}{achievable}\mspace{14mu}{sensitivity}\mspace{14mu}{based}\mspace{14mu}{on}\mspace{14mu}{the}}} \\\; & {{direct}\mspace{14mu}{measurement}\mspace{14mu}{of}\mspace{20mu}{calcium}\mspace{14mu}{resistance}\mspace{14mu}{is}\mspace{14mu} 0.2\mspace{11mu}\text{g}\text{/}{m^{2}.}} \\\; & \; \\\left. 7 \right) & {{{Water}\mspace{14mu}{vapour}\mspace{14mu}{transmission}\mspace{14mu}{rate}\mspace{11mu}({WVTR})} =} \\\; & {{0.2\mspace{11mu}\text{g/}m^{2} \times \left( {24\mspace{11mu}{hrs}\text{/}{{day} \div 4.2}\mspace{11mu}{hrs}} \right)} \approx} \\\; & \underset{\_}{1.1\mspace{11mu}\text{g/}{m^{2}.{day}}}\end{matrix}\mspace{11mu}$

Accordingly, the direct resistance method can also be used with thepresent sensor to calculate gas transmission rate. However, in thisspecific experimental setup, the direct resistance measurement method isnot suitable for determining a gas transmission rate of less than 10⁻⁶g/m²/day.

EXAMPLE 6 Calculation of Water Vapour Transmission Rate—Sensitivity ofResistance Measurements

The present example shows a method of deriving the water vapourtransmission rate based on sensitivity of resistance measurements usinga Keithley 24203A source meter. The data that is required forcalculation purposes were obtained from the measurements carried out inExample 1. The sensitivity of the source meter was specified to be about1 mΩ.

The derivation of water vapour transmission rate was carried out asfollows:

$\begin{matrix}\left. 1 \right) & {{The}\mspace{14mu}{change}\mspace{14mu}{in}\mspace{14mu}{calcium}\mspace{14mu}{sensor}\mspace{14mu}{thickness}\mspace{14mu}\left( {\Delta\; t} \right)\mspace{14mu}{with}\mspace{14mu}{respect}} \\\; & {{{to}\mspace{14mu} 1\mspace{11mu} m\;\Omega\mspace{14mu}{change}\mspace{14mu}{in}\mspace{14mu}{resistance}\mspace{14mu}\left( {\Delta\; R} \right)} =} \\\; & {\frac{\Delta\; R}{R} = {\frac{1\mspace{11mu} m\;\Omega}{{.37}\mspace{11mu}\Omega} = {{2.7 \times 10^{- 3}} = {\frac{\Delta\; t}{t} = {\frac{\Delta\; t}{150\mspace{11mu}{nm}} = {0.4\mspace{11mu}{nm}}}}}}} \\\; & \; \\\left. 2 \right) & {{Change}\mspace{14mu}{in}\mspace{14mu}{amount}\mspace{14mu}{of}\mspace{14mu}{Calcium}\mspace{14mu}({mol})\mspace{14mu}{for}\mspace{14mu}{the}\mspace{14mu}{change}\mspace{14mu}{in}} \\\; & {{1\mspace{11mu} m\;{\Omega.}} = {\frac{{Volume} \times {Density}}{A_{r}({Ca})} =}} \\\; & {\frac{0.4 \times 10^{- 9}m^{3} \times 1.55\mspace{11mu}\text{g/}10^{- 6}m^{3}}{40.08\;\text{g/}{mol}} =} \\\; & {1.55 \times 10^{- 5}\mspace{11mu}{mol}} \\\; & \; \\\left. 3 \right) & {{Two}\mspace{14mu}{water}\mspace{14mu}{molecules}\mspace{14mu}{are}\mspace{14mu}{required}\mspace{14mu}{to}\mspace{14mu}{react}\mspace{14mu}{with}\mspace{14mu}{one}\mspace{14mu}{calcium}} \\\; & {{{atom}.\mspace{11mu}{Hence}},{{the}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{water}\mspace{14mu}{molecules}}} \\\; & {{{required}\mspace{14mu}{to}\mspace{14mu}{react}\mspace{14mu}{with}\mspace{14mu} 1.55 \times 10^{- 5}\mspace{11mu}{mol}\mspace{14mu}{of}\mspace{14mu}{calcium}} =} \\\; & {{2 \times 1.55 \times 10^{- 5}\mspace{11mu}{mol}} =} \\\; & {3.1 \times 10^{- 5}\mspace{11mu}{mol}} \\\; & \; \\\left. 4 \right) & {{The}\mspace{14mu}{molecular}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu} H_{2}O\mspace{14mu}{is}\mspace{14mu} 18\mspace{11mu}\text{g/}{{mol}.}} \\\; & {{Hence},{{{the}\mspace{14mu}{weight}\mspace{14mu}{of}\mspace{14mu} 3.1 \times 10^{- 5}\mspace{11mu}{mol}\mspace{14mu}{of}\mspace{14mu} H_{2}O} =}} \\\; & {{3.1 \times 10^{- 5}\mspace{11mu}{mol} \times 18\mspace{11mu}\text{g/}{mol}} \approx} \\\; & {5 \times 10^{- 3}\mspace{11mu}\text{g/}m^{2}} \\\; & {{Therefore},{{the}\mspace{14mu}{sensitivity}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{digital}\mspace{14mu}{signal}\mspace{20mu}{analyser}\mspace{14mu}{for}}} \\\; & {{the}\mspace{14mu}{detection}\mspace{14mu}{of}\mspace{14mu}{water}\mspace{14mu}{vapour}\mspace{14mu}{permeation}\mspace{14mu}{is}\mspace{14mu}\underset{\_}{5 \times 10^{- 3}\mspace{11mu}\text{g/}{m^{2}.}}} \\\left. 5 \right) & {{{Water}\mspace{14mu}{vapour}\mspace{14mu}{transmission}\mspace{14mu}{rate}\mspace{14mu}({WVTR})} =} \\\; & {{5 \times 10^{- 3}\mspace{11mu}\text{g/}m^{2} \times \left( {24\mspace{11mu}{hrs}{\text{/day} \div 4.2}\mspace{11mu}{hrs}} \right)} \approx} \\\; & {2.9 \times 10^{- 2}\mspace{11mu}\text{g/}m^{2}\text{/day}}\end{matrix}$

Accordingly, the direct resistance method can also be used with thepresent sensor to calculate gas transmission rate. However, in thisspecific experimental setup, the sensitivity of resistance measurementswas not able to determine a gas transmission rate of less than 10⁶g/m²/day.

EXAMPLE 7 Estimation of Diffusion Coefficient

Extrapolating the curve in the steady state region of FIG. 10A to zeroprovides the lag time (L), which can be related to the elapsed timebefore steady state according to formula (V):

$L = \frac{l^{2}}{6D}$The barrier-coated polymer substrates are either assumed to be ahomogeneous single substrate or the oxide layer is considered to be thebarrier layer since the base polymer substrate provides only 3% ofresistance. The time lag technique is used to determine diffusioncoefficient. The necessary boundary conditions are the following: aninitially gas free film, attainment of equilibrium at the gas-polymerinterface, and zero concentration of gas at the calcium side. Underthese conditions, using the lag time L method, it is possible tocalculate the diffusion co-efficient for the required barrier film withthe following equation, where L is the lag time, l is the thickness ofthe barrier layer; D is the diffusion coefficient (m²/s).

The data needed for carrying out the present calculations is taken fromthe resistance and noise measurements from Example 2. The time taken tooxidise 150 nm of calcium was about 4.2 hours as shown in the graphs ofFIGS. 10A and 10B.

Therefore, the lag time (L) is 4.2 hours and silicon oxide coatedpolycarbonate barrier stack total thickness is 175 micron, hence, thediffusion coefficient (D) will be:

$\begin{matrix}{D = \frac{\left( {175\mspace{11mu}{\mu m}} \right)^{2}}{6 \times 4.2 \times 60 \times 60\mspace{11mu}\text{s}}} \\{= \frac{30625\mspace{11mu}({\mu m})^{2}}{90720\mspace{11mu} s}} \\{= {3.4 \times 10^{- 13}\mspace{11mu} m^{2}\text{/}\text{s}}}\end{matrix}$

Hence, the diffusion coefficient D for the given silicon oxide coatedsubstrate is 3.4×10⁻¹³ m²/s.

EXAMPLE 8 Estimation of Permeability Coefficient (P)

The rate of gas or vapour transfer through the membrane in the steadystate is usually calculated according to formula (VI):

${WVTR} = \frac{P_{\langle{p_{1} - p_{2}}\rangle}}{l}$where WVTR is water vapour transmission rate in given time [g/m².day], Pis permeability coefficient [g.μm/m².day.bar], p₁ is a pressure on thehigh pressure side of the barrier stack, p₂ is a pressure on the lowpressure side of the barrier stack, and l is path length (μm) fordiffusion (thickness of the barrier stack).

The data needed for carrying out the present calculations was taken fromthe resistance and noise measurements from Example 2. The time taken tooxidise 150 nm of calcium was about 4.2 hours as shown in the graphs ofFIGS. 10A and 10B.

The water vapour transmission rate calculated from the example 5 is 1.1g/m²/day. The total thickness of the barrier stack is 175 micron, p₁=55mbar (vapor pressure in the humidity chamber at 90% relative humidity),p₂=0 [vapour pressure inside calcium encapsulated test barrier stack isclose to zero since all the water vapor would react with calcium].

Applying the values into the above permeability coefficient equation:

$P = {{\left. \frac{{WVTR} \times l}{p_{1} - p_{2}}\Rightarrow\frac{192.5\mspace{11mu}{\text{g}.{\mu m}}\text{/}{m^{2}.\text{day}}}{{0.055{bar}} - 0}\Rightarrow{3.5 \times 10^{3}\mspace{11mu}{\text{g}.{\mu m}}\text{/}{m^{2}.{{day}\;@\mspace{11mu} 40}}{{{^\circ}C}.}} \right.\;\&}\mspace{11mu} 90\%\mspace{14mu}{RH}}$

Hence, the permeability coefficient (P) for the given silicon oxidecoated substrate is 3.5×10³ g.μm/m².day at one bar with 40° C. & 90% RHconditions.

EXAMPLE 9 Study of the Effect of a Liner Layer on Sensor DegradationPattern

In the present example, test were carried out on polymer substrates tostudy the effect which a liner layer has on the degradation pattern ofthe sensing element.

A sensor comprising a calcium sensing element was fabricated on a PETsubstrate which had a metal oxide barrier coating according to theprocedure described in Example 1. A 100 nm thick acrylic polymer linerwas applied onto the surface of the metal oxide coating. The sensor wasencapsulated in a similar way to an OLED package, as shown in FIG. 6. Acontrol was fabricated without incorporating an liner layer. The test onboth test specimens was carried out in a humidity chamber at 50° C.temperature and 90% relative humidity.

The images as shown in FIGS. 11A and 11B were taken at intervals ofseveral hours into the test. Calcium degradation was monitored throughan optical microscope and digital images were taken from five locationson the surface of the sensing element. FIG. 11A shows the uniformdegradation of the sensing element as the test progressed. Incomparison, FIG. 11B shows that the sensing element in the control wasdegraded non-uniformly.

From these results, it can be seen that a uniform, layer-by-layerdegradation of the sensing element in the sensor was facilitated by theliner layer.

EXAMPLE 10 Study of the Effect of a Non-Inorganic Terminating Layer onSensor Degradation Pattern When Test Material is Glass

In the present example, a test using a sensor of the invention wascarried out on a glass substrate. The purpose is to study the uniformityof degradation in the sensor.

A sensor was fabricated onto a glass substrate according to theprocedure described in Example 1. No liner layer was applied onto thesurface of the glass substrate. The sensor was designed to have asimilar layout as an OLED package structure shown in FIG. 6. The sensorwas encapsulated with adhesive material and a glass cover substrate. Thetest was carried out in a humidity chamber at 50° C. temperature and 90%relative humidity. As water vapour penetrated through the adhesivematerial, the sensing element was gradually degraded. The sensor wasmonitored through an optical microscope. Digital images were taken from5 defined locations for each measurement at intervals of several hours(cf. FIG. 12A). The images show that the degradation of the sensingelement is uniform in an OLED package.

The optical properties of the calcium sensor were monitored using aUV-3101 PC UV-VIS-NIR scanning spectrophotometer from SHIMADZU. FIG. 12Bshows the graph of reflectivity of the calcium sensor as measured atintervals of several hours on 5 different locations on the sensingelement. It can be seen from the graph that the measured reflectance ateach location of sensor remains same and does not show any substantialvariation. Therefore, calcium oxidation appears to be uniform across thesensor and the drop in reflectance confirms that the dynamics of calciumoxidation occurs in a layer-by-layer progression.

It can be seen that the calcium sensing element fabricated using an OLEDpackage structure is oxidized uniformly and in a layer-by-layerprogression. Accordingly, the decrease in electrical conductivity of thesensor can be correlated to the corresponding decrease in thickness ofthe sensing element.

EXAMPLE 11 Measurement of Combined Oxygen and Water Vapour PermeationProperties Through a Test Material

The present invention can be carried out not only in a humidity chamberbut also in a custom made test cell. The present invention allows themeasurement of permeation of oxygen, water vapour, or any other gas,through any test substrate or package can be carried out in a singletest chamber. In addition, the measurement can be carried out atelevated temperatures and pressures.

FIG. 13 depicts a test cell which comprises a conventional gas chamberprovided with 2 pairs of gas inlets and outlets. The chamber comprises atest specimen clamping section located at the middle portion of thechamber. In order to carry out a measurement in the test chamber, thetest material is first cut according to required the dimensions. Thesensor is fabricated without an encapsulation onto the test materialaccording to a method described above. When the test specimen isinserted into the chamber and clamped at the clamping section, thechamber is divided into 2 portions, A and B. A gas mixture having acomposition that simulates the required test conditions is prepared andchannelled into chamber A. The gas mixture enters the chamber via theinlet and is evacuated via the outlet. If necessary, Chamber B isprovided with an inert environment by circulating dry nitrogen gas intothe chamber. This may also serve to flush out any reactive gas thatpermeates into Chamber B. A suitable RH probe monitors the relativehumidity.

A suitable electrical feed-through can be used to connect the conductivetrack to measure resistance and 1/f noise spectrum. The water or oxygenmolecule that permeated through low gas permeability test substratereacts with the calcium sensor. Constant source meter and digital signalanalyzer can measure the resistance and 1/f noise spectrum respectively.The rate of change of resistance and noise characteristics can be usedto calculate water vapour or oxygen transport properties of the testfilm.

What is claimed is:
 1. A sensor for measuring gas permeability of a testmaterial, comprising: an electrically conductive sensing element thatcomprises a water and/or oxygen sensitive material, wherein the reactionof said material with water or oxygen when the sensing element iscontacted with water and/or oxygen results in a change in the electricalconductivity of the sensing element, wherein the water and/or oxygensensitive material is selected from the group consisting of metals ofGroup I of the periodic system of elements, metals of Group II of theperiodic system of elements, iron, tin, chromium, conductive polymers,and mixtures and combinations thereof; two electrodes electricallyconnected to the sensing element; a base substrate that supports thesensing element, wherein the electrodes are located on a surface of thesubstrate, wherein the electrodes are spaced apart, thereby forming atrench, wherein edges of the trench are defined by the edge of eachelectrode and the base of the trench is the base substrate, wherein thesensing element is located in the trench; and a liner layerinterdisposed between the sensing element and the base substrate,wherein the liner layer adjoins the sensing element, and wherein theliner layer comprises an organic polymer and/or an inorganic polymer. 2.The sensor of claim 1, wherein the electrodes provide electricalconnection between the sensing element and an electrical signalevaluation means.
 3. The sensor of claim 1, wherein the water and/oroxygen sensitive material is selected from the group consisting ofcalcium and magnesium.
 4. The sensor of claim 1, wherein the conductivepolymer is selected from the group consisting of polyaniline,polypyrrole and polythiophene, polyacetylene, poly-p-phenylene, andpolyvinylpyridine, thiophene-bipyridine copolymers, polypyridine,polybipyridine, and organometallic polyphenylenes.
 5. The sensor ofclaim 1, wherein the electrodes comprise an electrically conductivematerial selected from the group consisting of a metal, metal oxide andmixtures and combinations thereof.
 6. The sensor of claim 5, wherein themetal is selected from the group consisting of silver, gold, aluminiumand copper.
 7. The sensor of claim 5, wherein the metal oxide isselected from the group consisting of indium tin oxide, aluminium zincoxide, and indium zinc oxide.
 8. The sensor of claim 7, wherein the basesubstrate comprises a polymeric material.
 9. The sensor of claim 8,wherein the polymeric material comprises an organic polymer selectedfrom the group consisting of polycarbonate, polyethylene,polythersulfone, epoxy resins, polyethylene terephthalate, polystyrenes,polyurethanes and polyacrylates.
 10. The sensor of claim 8, wherein thepolymeric material comprises an inorganic polymer selected from thegroup consisting of silicones, polydimethylsiloxanes,biscyclopentadienyl iron, polydichlorophosphazene and derivativesthereof.
 11. The sensor of claim 7, further comprising a barrier layerformed on the base substrate.
 12. The sensor of claim 11, wherein thebarrier layer comprises a material selected from the group consisting ofmetals, metal oxides, ceramic oxides, inorganic polymers, organicpolymers and mixtures and combinations thereof.
 13. The sensor of claim1, further comprising an encapsulation enclosing the sensing element.14. The sensor of claim 13, wherein the encapsulation comprises apolymeric material selected from the group consisting of epoxy polymers,polysulfide, silicone and polyurethane.
 15. The sensor of claim 14,wherein the encapsulation provides a hollow space around the sensingelement.
 16. The sensor of claim 15, wherein the hollow space is filledwith an inert gas.
 17. The sensor of claim 13, further comprising acover substrate, wherein the encapsulation is formed as side (lateral)walls surrounding the sensing element, and the cover substrate isarranged to be in contact with the side (lateral) walls.
 18. The sensorof claim 17, wherein the cover substrate comprises a material selectedfrom the group consisting of glass, aluminium and copper.
 19. The sensorof claim 1, further comprising a protective layer covering at least aportion of the sensing element.
 20. The sensor of claim 19, wherein theprotective layer comprises a material selected from the group consistingof a metal, a metal alloy, a metal oxide, a metal oxide mixture, a metalfluoride and an organic polymer.
 21. The sensor of claim 20, wherein themetal fluoride is selected from the group consisting of LiF and MgF₂.22. The sensor of claim 1, wherein the organic polymer is substantiallypermeable to gas.
 23. The sensor of claim 1, wherein the organic polymeris selected from the group consisting of acrylic polymers, and parylenetype polymers.
 24. The sensor of claim 1, wherein the inorganic polymercomprises a silicone-based polymer.
 25. A method of producing a sensorfor measuring gas permeability of a test material, said methodcomprising: providing a base substrate that supports a sensing elementand that further comprises a liner layer, wherein the liner layercomprises an organic polymer and/or an inorganic polymer; depositing onthe liner layer an electrically conducting sensing element thatcomprises a water and/or oxygen sensitive material selected from thegroup consisting of metals of Group I of the periodic system ofelements, metals of Group II of the periodic system of elements, iron,tin, chromium, conductive polymers, and mixtures and combinationsthereof, so that the liner layer is interdisposed between the basesubstrate and the sensing element and the liner layer adjoins thesensing element; providing two electrodes, wherein the electrodes arelocated on a surface of the substrate, wherein the electrodes are spacedapart, thereby forming a trench, wherein edges of the trench are definedby the edge of each electrode and the base of the trench is the basesubstrate, wherein the sensing element is located in the trench; andconnecting the electrically conductive sensing element to said pair ofelectrodes.
 26. A system for measuring the gas permeability of a testmaterial, said system comprising a sensor for detecting moisturepermeation through the test material, said sensor comprising: anelectrically conductive sensing element that comprises a water and/oroxygen sensitive material selected from the group consisting of metalsof Group I of the periodic system of elements, metals of Group II of theperiodic system of elements, iron, tin, chromium, conductive polymers,and mixtures and combinations thereof, wherein the reaction of saidmaterial with water or oxygen when the sensing element is contacted withwater and/or oxygen results in a change in the electrical conductivityof the sensing element; two electrodes electrically connected to thesensing element, wherein the electrodes provide electrical connectionbetween the sensing element and an electrical signal evaluation means; abase substrate that supports the sensing element, wherein the electrodesare located on a surface of the substrate, wherein the electrodes arespaced apart, thereby forming a trench, wherein edges of the trench aredefined by the edge of each electrode and the base of the trench is thebase substrate, wherein the sensing element is located in the trench;and a liner layer interdisposed between the sensing element and the basesubstrate, wherein the liner layer adjoins the sensing element, andwherein the liner layer comprises an organic polymer and/or an inorganicpolymer.
 27. A method of determining the gas permeability of a testmaterial using a sensor for measuring gas permeability of the testmaterial, said sensor comprising: an electrically conductive sensingelement that comprises a water and/or oxygen sensitive material selectedfrom the group consisting of metals of Group I of the periodic system ofelements, metals of Group II of the periodic system of elements, iron,tin, chromium, conductive polymers, and mixtures and combinationsthereof, wherein the reaction of said material with water or oxygen whenthe sensing element is contacted with water and/or oxygen results in achange in the electrical conductivity of the sensing element; twoelectrodes electrically connected to the sensing element, wherein theelectrodes provide electrical connection between the sensing element andan electrical signal evaluation means; a base substrate that supportsthe sensing element, wherein the electrodes are located on a surface ofthe substrate, wherein the electrodes are spaced apart, thereby forminga trench, wherein edges of the trench are defined by the edge of eachelectrode and the base of the trench is the base substrate, wherein thesensing element is located in the trench; and a liner layerinterdisposed between the sensing element and the base substrate,wherein the liner layer adjoins the sensing element, and wherein theliner layer comprises an organic polymer and/or an inorganic polymer;wherein said method comprises: contacting the sensing element with waterand/or oxygen; measuring the changes in electrical conductivity of thesensing element over a period of time; and calculating the gaspermeability coefficient of the test material based on the measurements.28. The method of claim 27, further comprising measuring the change in1/f type noise spectrum density over the period of time.